Changing climate and high-severity wildfire are causing widespread tree mortality across the southwestern United States, significantly reducing the seed source availability to support post-fire regeneration of forest and woodland. This presents a challenge to both forest regeneration, and the potential contribution of disturbed areas to carbon storage. Planting tree seedlings can accelerate reforestation and rates of carbon sequestration, but in the semi-arid southwestern US, the carbon storage potential of reforestation efforts is uncertain due to limited information on seedling growth and survival rates. We surveyed eight post-wildfire reforestation sites and developed allometric equations to predict carbon sequestration as a function of directly measured seedling size, and indirectly using years since planting and current and future climate scenarios. Our findings highlight the critical role of seedling survival in the first few years after planting, which dramatically alters carbon stock trajectories. Future climate conditions, characterized by higher temperatures at low-elevation sites, may hinder reforestation efforts and reduce the carbon storage potential of these areas. Given our findings, we recommend climate-informed planting strategies in post-wildfire areas, including adjusting planting densities to maximize carbon stock accumulation while ensuring future wildfire resiliency. This research offers practical insights for forest managers aiming to improve reforestation outcomes, particularly as we face the challenges of a changing climate.